1. Field of the Invention
This invention relates to projection optical systems, more particularly to projection optical systems with variable magnification.
2. Background of the Invention
Projection displays have several constraints that present obstacles to designing their optical system. These typically require low distortion for clearer viewing, wide field angles to present a clear picture to a large number of viewers in a given area, high resolution for picture quality, high light throughput for brightness and uniform screen illumination. The use of reflective spatial light modulators add further constraints to the design.
Spatial light modulators, in this context, are arrays of individually addressable elements, where each element typically represents one picture element, or pixel, in the viewed image. Reflective spatial light modulators will often comprise arrays of these elements, each of which is capable of reflecting light to the display surface or away from the display surface. Transmissive modulators, in contrast, typically lie directly in the light path, either transmitting or blocking the light as addressed.
Some problems with illumination efficiency and artifacts can be eliminated with reflective modulators by using offset projection. Typically, the modulators will be lined up to be on the axis of the center of the projection lens. However, this can lead to a distortion referred to as keystoning. Keystoning arises when the projection axis is not perpendicular to the screen. For example, a projector sitting on a table is tilted upward to project an image which is entirely above the table level. Because the top of the image is farther from the projector than the bottom of the image, the upper comers of the image are farther apart than the lower corners. By moving the modulator off of the axis of the projection lens, this result can be avoided.
In addition to all of these requirements, reflective modulators require long back focal lengths. Light impinges the modulator and is formed into an image as the modulator reflects the light off of the modulator array. The incoming light path and the reflected light path must stay separate from each other, or the image will be affected. This separation can be accomplished by increasing the amount of space between the incoming light path and the outgoing light path, which results in a long back focal length.
Finally, it is often desirable to have variable magnification, as in a zoom lens, in a projector that can meet all of the above requirements. Therefore, some optical system is needed that meets the requirements of low distortion; wide field capability; uniform screen illumination; zoom capability and offset projection.